1. Field
The present invention relates to a switching power supply with quick transient response against abrupt changes of load current, in particular to a switching power supply with variable output voltage and quick transient response, which is helpful for improving transient response rate of switching power supply, reducing fluctuation amplitude of output voltage, and reducing the time required for stabilizing the output voltage.
2. Description of the Related Art
Nowadays, as microprocessors and DSPs (Digital Signal Processors) develop, there is a demand for switching power supply with high current variation rate and quick transient response. In fact, microprocessors and DSPs are rapid-changing loads for switching power supply. For instance, to reduce power consumption, in idle state, microprocessors and DSPs enter into “Sleep Mode”, in which the working current is very low; once needed, microprocessors and DSPs create instantaneous high current; thus, the switching power supply must have capability of quick transient response.
LnLC (Linear non-linear Control) is a method for improving the transient response rate for switching power supply, by adding a non-linear control loop on the basis of the existing linear control loop. When the load current is stable, the linear control loop of the switching power supply functions; once the load current changes abruptly and results in an output voltage variation beyond the limit, the non-linear control loop takes the place of the linear control loop and increases the transient response rate. Such a non-linear control loop requires two comparators to detect whether the output voltage is lower than the lower limit or higher than the upper limit. For a switching power supply unit with variable output voltage, additional circuits are required to produce limit which changes by following the output voltage, as a result of which, the circuit complexity and chip area are increased. A satisfactory transient response improving circuit must be capable of quickly stabilizing the output voltage, and preferably has simple structure and are less influenced by manufacturing technique and working environment.